Polyethylene terephthalate (“PET”) is used extensively in packaging applications, in particular as beverage containers. In these applications, it is important that the PET have a relatively high molecular weight, generally expressed as inherent viscosity (“Ih.V.”) or intrinsic viscosity (“It.V.”), and low amounts of acetaldehyde (AA). Acetaldehyde has a noticeable taste and can be detected by human taste buds at low levels. When preforms are blown into bottles, unacceptably high AA levels adversely impact the taste of the beverage contained in said bottles.
The conventional PET production process begins with esterification of predominantly terephthalic acid and ethylene glycol, or ester exchange of predominantly dimethyl terephthalate and ethylene glycol. The esterification need not be catalyzed. Typical ester exchange catalysts, which may be used separately or in combination, include titanium alkoxides, tin (II) or (IV) esters, zinc, manganese or magnesium acetates or benzoates and/or other such catalyst materials that are well known to those skilled in the art. The resulting mixture is then subjected to polycondensation in the melt at elevated temperature, for example 285° C., in the presence of a suitable catalyst. Compounds of Sn, Sb, Ge, Ti, or others have been used as polycondensation catalysts.
Following melt phase polycondensation, which generally achieves an inherent viscosity in the range of 0.5 to 0.65, the polyester is extruded, cooled, and cut into granules, which are then subjected to a crystallization process wherein at least the exterior of the granules becomes crystalline. This crystallinity is necessary to prevent sintering and agglomeration in a subsequent solid state polymerization. Crystallization and annealing take place in a fluidized bed at temperatures of, for example 160-220° C., for several hours, as discussed by WO 02/18472 A2, and U.S. Pat. Nos. 4,161,571; 5,090,134; 5,114,570; and 5,410,984.
Solid state polymerization or “solid stating” is performed to increase the intrinsic viscosity of the polymer in the solid state. Volatiles such as acetaldehyde are removed in vacuo or by a flow of inert gas (e.g., nitrogen) in solid state polymerization operations.
Solid stating has the advantage that relatively high inherent viscosities can be achieved. It has the further advantage that free acetaldehyde content of the polymer is lowered substantially by the removal of free acetaldehyde by volatilization. Solid stating has the considerable disadvantages of high energy usage and long manufacturing time. Finally, solid state polymerization causes the pellets to develop shell-to-core molecular weight gradients, which results in a loss in inherent viscosity during the molding of articles; the loss in Ih.V. is theorized to be due to re-equilibration in the melt.
JP 4879896 disclosed a method for producing polyester wherein bis-(β2-hydroxyethyl)terephthalate or a mixture of bis-(β2-hydroxyethyl)terephthalate and at least one other bifunctional compound is polycondensed in the presence of a titanium compound catalyst to produce a polyester with a high degree of polymerization so that at least 85% of the repeating structural units of the polyester are ethylene terephthalate units, and wherein a phosphorus compound is added to the molten polyester once the polycondensation reaction has been completed.
It would be desirable to eliminate solid stating, but to do so would require more extended melt-phase polycondensation, that is, longer times at temperatures above the melting point of the polyester. With all other parameters being equal, the amount of free AA generated in the melt phase manufacture and the number of AA precursors made in the melt phase manufacture increase dramatically as the It.V. (or molecular weight) of the polymer increases. Furthermore, as the It.V. increases it becomes more difficult to remove the free AA from the higher viscosity melt. Molecular weight build-up in the melt has until recently typically been limited to a reasonably low number (e.g., It.V. of about 0.65 dL/g or less, more usually between 0.55 and 0.60 dL/g or less), followed by further advancing the molecular weight of the polymer in the solid state.
There are several causes for the formation of free AA and AA precursors; the AA precursors which contribute to additional AA generated upon melting the solid polyester particles in subsequent melt processing such as during injection molding of bottle preforms. One contributor to the formation of free AA and AA precursors during melt phase polymerization is the thermal degradation of the polyester polymers in the melt phase which becomes more prevalent as the It.V. of the polymer is increased at high temperatures. When solid-state polymerization is not used to increase the molecular weight, a longer melt-phase residence time may be necessary to produce the molecular weight needed to later mold acceptable preforms, which can be blown subsequently into bottles having the required properties for a given application. This extended melt-phase exposure increases the extent of thermal degradation; therefore, producing PET exclusively in the melt phase with acceptable free AA and/or acceptable AA generation rate during subsequent molding is much more challenging than the conventional scenario where a portion of the molecular build-up occurs in a solid-phase process. Along with a shorter melt-phase step which generates less free AA and fewer AA precursors, conventional processes have the added advantage of the solid-stating gas sweeping away most of the free AA (also described herein as “residual AA”) contained on or in the solid polyester particles.
Another contributor is inadequately stabilized and/or deactivated polycondensation catalyst used in the melt phase which can, during the melting of solid polyester particles in a melt processing zone, continue to catalyze the conversion of AA precursors, which are present in the polymer due to thermal degradation, to form free AA during subsequent melt processing to form the article. Adequately stabilizing and/or deactivating the polycondensation catalyst, therefore, reduces the amount of free AA generated during melt processing to form the article (reduces the AA generation rate), even though AA precursors may be present in the melt. While catalyst stabilization and/or deactivation does reduce the free AA generated in melt processing steps, some free AA is nevertheless generated. It is theorized that there may be an uncatalyzed route for conversion of precursors to free AA or that a lower level of catalytic activity may remain to convert some of the AA precursor species to free AA or that acid catalysis of AA precursors to free AA occurs or that some combination of 2 or more of the previous 3 options occurs; however, the invention is not bound by theory. Moreover, the ease to which catalyst metals can be deactivated differs from metal to metal. For example, Ti metal based catalysts can be deactivated with phosphorus compounds, for example, phosphate compounds
The problem of controlling the presence of free AA and AA precursors produced in the melt-phase manufacture was discussed in EP 1 188 783 A2, equivalent to U.S. Pat. No. 6,559,271 B2. This patent proposes that the amount of free AA and AA precursors can be limited by keeping the reaction temperature during the entire polycondensation step below 280° C., by using a low dosage of a highly active titanium catalyst to limit the residence time of the polymer in the melt-phase manufacture, and by using an excess of AA scavenger added in the melt phase manufacture. To control AA generation from AA precursors produced in the melt phase manufacture, this patent teaches deactivating the catalyst with a phosphorus compound late toward or after the end of polycondensation so as to allow the catalyst to promote the molecular weight build-up to a intrinsic viscosity (It.V.) of 0.63 dL/g and higher. Finally, the amount of the AA scavenger or binder added must be in excess so as to bind not only the residual or free AA produced in the melt phase manufacture, but to also bind whatever free AA is generated in subsequent melt processing steps.
One problem with the approach of using an acetaldehyde scavenger is that they are expensive regardless of when they are added. Another problem of adding acetaldehyde scavengers to the melt phase manufacture is that a portion of the scavenger is consumed by the free acetaldehyde present in the melt phase manufacture, thereby requiring the addition of an excess amount of scavenger to bind subsequently formed acetaldehyde. As the amount of acetaldehyde scavenger added in the melt phase manufacture increases, so do costs and the degree of yellow hue imparted to the polymer by the scavenger, especially if a class of scavengers containing amine groups is used, like low molecular weight polyamides. The presence of some acetaldehyde scavengers may also lead to an increased concentration of black specks in the polyester particles or in the molded part. Moreover, the effectiveness of the scavenger may be impaired by undergoing two heat histories where the polyester is molten, especially when one of the heat histories is under high vacuum, high temperature, and high viscosity conditions (as in the melt phase polycondensation) where the thermal stability of some types of scavenger may be compromised and may be lost due to scavenger volatility. With some scavengers, the amount of yellow color imparted by the scavenger may increase as the number of melt heat histories increases. It would be desirable, therefore, to produce solid high IV polyester polymer particles which do not contain acetaldehyde scavengers added in the melt phase yet have both a low AA generation rate and low residual acetaldehyde levels when fed to a subsequent melt processing zone.
U.S. Pat. No. 5,898,058 discloses using any one of a large number of conventional polycondensation catalysts (with combinations of Sb catalysts and one of Co, Zn, Mg, Mn or Ca based catalysts exemplified and/or claimed) in which the catalysts are deactivated late. This patent notes that the traditional antimony polycondensation catalyst will begin to catalyze or encourage the degradation of the polymer, leading to the formation of acetaldehyde and yellowing of the polymer. Once the polycondensation reaction essentially reaches completion, further reaction allows the catalyst to degrade the polymer and form acetaldehyde and a yellow hue. The patent discloses the manufacture of polyester precursors at an It.V. of about 0.64 and 0.62 dL/g, or 0.60 dL/g which was increased to an It.V. of 0.81 dL/g by solid state polymerization. The patent notes that solid state polymerization techniques are useful to increase the It.V. of the polyester to these higher levels.
U.S. Pat. No. 5,656,716 discloses use of high surface area titanium catalysts followed by addition of triphenyl phosphate. Without the triphenyl phosphate, a high inherent viscosity but distinctly yellow product was obtained. With triphenyl phosphate, less colored products were obtained, but only at a low inherent viscosity, thus requiring solid stating of these products with its disadvantages.
It would be desirable to be able to produce PET and other polyesters with an inherent viscosity suitable for production of food and beverage containers without the necessity for solid stating, and with a lower content of acetaldehyde in the absence of organic AA scavengers, and/or with reduced levels of acetaldehyde generated during subsequent melt processing in the absence of organic AA scavengers. It would further be desirable to produce PET in shorter reaction time, due to a more active catalyst than antimony, while maintaining or improving upon the AA properties of the product, with or without solid state polymerization.